The primary objective of this study is to evaluate the absolute bioavailability of a single, fixed subcutaneous (SC) dose of aducanumab compared with a single weight-based intravenous (IV) dose of aducanumab in healthy volunteers.

The secondary objectives of this study are to assess the safety and tolerability of aducanumab administered SC in healthy volunteers and to characterize additional pharmacokinetic (PK) parameters of a single, fixed SC dose of aducanumab and a weight-based IV dose of aducanumab in healthy volunteers.

Key Inclusion Criteria:

• Have a body mass index (BMI) between 18 and 30 kilograms per meter square (kg/m^2), inclusive.
• Have a negative test result for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on Day -1.

Key Exclusion Criteria:

• History of any clinically significant cardiac, endocrine, gastrointestinal, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, or renal disease, or other major disease, as determined by the investigator.
• History of severe allergic, anaphylactic, or systemic hypersensitivity reactions, or of any allergic reactions that in the opinion of the investigator are likely to be exacerbated by aducanumab, the excipients contained in the formulation, and if appropriate, and diagnostic agents to be administered during the study.
• Chronic, recurrent, or serious infection (e.g., pneumonia, septicemia), as determined by the investigator, within 90 days prior to screening or between screening and Day -1.
• Any immunization or vaccination given within 10 days prior to Day 1 and for 10 days after Day 1.
• Current enrollment in any other drug, biological, device, or clinical study, or treatment with an investigational drug or approved therapy for investigational use within 30 days prior to Day -1, or 5 half-lives, whichever is longer.
• Prior exposure to the study treatment or previous participation in this study or previous studies with aducanumab.
• Mini mental state examination (MMSE) score of <27 at screening.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/